Optical computing, optical switching, and optical interconnection are three emerging technology areas which depend on the ability to modulate optical beams. In several of theses areas, most interest is focussed on the semiconductor devices and arrays which operate on light beams propagating perpendicular to the surface plane of the devices or arrays. Such devices are commonly called "surface-normal" devices. These devices are relatively compact which, in turn, leads to simple and efficient coupling to the devices.
A recent class of surface normal devices has been developed in which a p-i-n diode structure is realized with one or more quantum wells located usually in the intrinsic region between the p and n contact regions. One example of a high speed, surface normal optical modulator employing semiconductor quantum wells is shown in U.S. Pat. No. 4,525,687. This particular type of modulator employs electro-absorption to perform optical modulation. Optical beams incident normal to the modulator surface are either absorbed within the quantum well region of the modulator or permitted to pass through the modulator without significant absorption. Absorption and, thereby, modulation are controlled by electrical signals applied to the modulator.
When a modulator absorbs photons from the incident optical beam, electron-hole pairs are generated. The presence of these carrier pairs disturbs the absorption characteristics of the quantum well region in the modulator. Motion of the carriers through the modulator under the influence of applied electric fields results in ohmic heating and increases the power dissipated in the modulator, potentially to values significantly exceeding the absorbed optical power.
Heating within the modulator produces a shift of the semiconductor quantum well absorption edge to a new wavelength. This shift results in a misalignment of the absorption wavelength and the incident optical beam wavelength so that modulation ceases or is severely affected. Thus, ohmic heating limits the maximum optical power which can be modulated.
Photo-generated electrical charge carriers also tend to screen applied electric fields, which hampers or even inhibits the modulation process when carrier populations are sufficiently high. Moreover, carrier production reduces the optical absorption coefficient of the quantum well material by exciton bleaching and, thereby, makes this type of modulator unattractive for higher optical beam intensity applications.
The problems caused by these effects within the absorption modulators have detracted to some degree from the initial appeal of the devices and have caused device designers to favor other electro-optic effects such as electrore-fraction for realizing satisfactory optical modulators. There has been no known effort to alleviate the problems associated with the electro-absorption, quantum well modulators.